1. Field of the Invention
The present invention relates to an ArF excimer laser and a scanning type exposure device. And, the invention also relates to an ultraviolet laser device which oscillates pulsed laser by adding a trace quantity of xenon gas to the gas for the ultraviolet laser introduced into a chamber and exciting the gas for the ultraviolet laser by the pulse oscillation in this chamber, and particularly to an ultraviolet laser device which efficiently supplies the xenon gas into the chamber without modifying an existing laser gas supply equipment.
2. Description of the Related Art
It is conventionally known that the ArF excimer laser uses argon (Ar) and fluorine (F2) as a laser gas for a light source of the exposure device which performs exposure of semiconductors. FIG. 7 shows a cross sectional view of the structure of ArF excimer laser 1 according to prior art. The ArF excimer laser 1 of FIG. 7 has laser chamber 2 in which laser gas is sealed and an electric discharge is caused to oscillate laser light 11 and narrow-banding unit 10 for narrow-banding (precisely controlling a spectrum width and a center wavelength) the laser light 11 oscillated by the laser chamber 2.
F2, Ar and neon (Ne) are enclosed as the laser gas at a predetermined pressure ratio in the laser chamber 2. Among them, Ne is a buffer gas occupying about 98% or more of the total volume of the laser gas.
The chamber 2 has therein flowing fan 19 which is rotated by motor 20 therein to circulate the laser gas in the chamber 2 so to guide to between discharging electrodes 5 and 5. And, a high voltage is applied to between the discharging electrodes 5 and 5 from high-voltage power supply 18 to excite the laser gas by the electric discharge so to oscillate the laser light 11 having a wavelength of about 193 nm.
The oscillated laser light 11 is narrow-banded in the narrow-banding unit 10 disposed outside of the back of the laser chamber 2 and entered a step and repeat exposure device 15 (hereinafter called the stepper) to serve as a light source to expose semiconductor chips.
Generally, in the ArF excimer laser 1 of the above type, a high voltage is applied in a pulsed pattern to perform pulse oscillation of the laser light 11. Concerning the pulse oscillation, output per pulse is determined as pulse output, and an oscillation frequency of the laser light 11 is determined as a pulse frequency.
Excitation of the laser gas by an electric discharge undergoes the following process. Specifically, the buffer gas to which energy was given by the electric discharge collides with the molecules of F2 and Ar to give energy to F2 and Ar so to produce ArF excimer molecules. The energy of the ArF excimer molecules is discharged to cause the laser oscillation.
In the KrF excimer laser, helium (He) had been used for the buffer gas before. But, it came to be known that when Ne was used as the buffer gas, efficiency to give energy to the molecule of F2 and Ar was higher as compared with the use of He, and the pulse output was increased. Therefore, Ne was originally used as the buffer gas in the ArF excimer laser 1.
FIG. 8 shows an explanatory diagram of the stepper 15. In FIG. 8, the stepper 15 has reticle 38 (mask for exposure) which is an expansion original plate of an IC circuit, wafer stage 43 which has thereon wafer 41 for producing semiconductor chips and is freely movable in X and Y directions in the drawing, lighting lenses 37 for irradiating the laser light 11 to the reticle 38, and projection lens 39 which gathers the laser light 11, which has passed through the pattern of the reticle 38, to irradiate it to the semiconductor chips on the wafer 41.
The laser light 11 having entered the stepper 15 is shaped by the lighting lenses 37 and irradiated to substantially the entire surface of the reticle 38. The laser light 11 having passed through the reticle 38 is irradiated to the entire area of one of the semiconductor chips for a fixed number of pulses through the projection lens 39 to perform the exposure of the semiconductor chip by one operation. After completing the exposure of one semiconductor chip, the wafer stage 43 is moved to perform the exposure of the next semiconductor chip.
But, the aforesaid prior art has the following disadvantages.
The number of semiconductor chips that the stepper 15 can expose in each unit time (hereinafter called the stepper capability) increases as the power (product of pulsed output and pulse frequency) of the laser light 11 increases. In other words, time required for irradiating one semiconductor chip is shortened by increasing the pulse output.
However, there is a problem that a current-carrying capacity of the high voltage power supply 18 must be made large to increase the pulse output, resulting in upsizing the ArF excimer laser 1.
Moreover, it is necessary to increase a flow velocity of the laser gas which flows between the discharging electrodes 5 and 5 to raise the pulse frequency. It is because a trouble is caused in excitation of the laser gas and the pulse output is lowered greatly if the laser gas deteriorated by a pulse discharge is not removed from between the discharging electrodes 5 and 5 before the next pulse discharge.
However, there is also a problem that the ArF excimer laser 1 is upsized because it is necessary to enlarge the flowing fan 19 to increase a flow rate of the laser gas and to enlarge the motor 20 so to raise the number of rotations.
Now, the ultraviolet laser device used for conventional exposure of semiconductors will be described.
Conventionally, a semiconductor exposure device having the ultraviolet laser device, such as an excimer laser device, as a light source repeats exposure and stage movement alternately to make exposure of IC chips on a semiconductor wafer. Therefore, the ultraviolet laser device performs a burst operation to repeat a continuous pulse oscillation operation which continuously causes a pulse oscillation of the laser light for a given number of times and a pause of the oscillation to pause the pulse oscillation for a predetermined duration.
But, when the burst operation is performed, a burst characteristic and a spike characteristic that energy is gradually lowered are caused, and the laser output from the excimer laser device has a change in energy at every burst. As a result, there is a disadvantage that an amount of light exposure is variable.
Therefore, the applicant for the present invention has proposed a technique to improve variations in the amount of light exposure due to the burst characteristic and the spike characteristic by adding a trace quantity of xenon gas to the gas for ultraviolet laser in the chamber in Japanese Patent Application No. 11-23709.
This art is not effective because it is necessary that a xenon gas cylinder for supplying xenon gas into the chamber is separately disposed from a gas cylinder for the ultraviolet laser and the xenon gas is fed into the chamber three passage quite different from a passage for feeding the gas for ultraviolet laser.
Especially, since a site where the excimer laser is set up is provided with only the laser gas supply equipment such as a gas cylinder for the ultraviolet laser inherently required, it is not efficient to modify such equipment only to supply the xenon gas into the chamber.
Accordingly, in order to remedy the burst characteristic and the spike characteristic of the ultraviolet laser device by adding a trace quantity of xenon gas, it is quite significant to efficiently supply the xenon gas into the chamber without modifying the existing laser gas supply equipment.
The present invention was completed in view of the above-mentioned problems. And, it is a first object of the invention to provide ArF excimer laser capable of raising a pulse frequency without involving the enlargement of the device and an exposure device which has the ArF excimer laser of a high pulse frequency as the source of light.
To solve the above disadvantages and to remedy the burst characteristic and the spike characteristic of the ultraviolet laser device by adding a trace quantity of xenon gas, it is a second object of the present invention to provide an ultraviolet laser device which can efficiently supply xenon gas into the chamber without modifying the existing laser gas supply equipment.
To achieve the aforesaid first object, a first aspect of the invention is an ArF excimer laser which causes an electric discharge between discharging electrodes to excite a laser gas and oscillates a narrow-banded laser light, wherein a buffer gas contained in the laser gas mainly consists of He.
According to the first aspect of the invention, the buffer gas for the ArF excimer laser mainly consists of He. Compared with Ne conventionally used as the buffer gas, He has a density of about ⅕, the gas flow velocity between the discharging electrodes becomes 2.2 times the conventional one by using He as the buffer gas because the gas flow velocity of the flowing fan 19 is in inverse proportion to the square root of the density of the gas. Thus, the laser light having a high pulse frequency can be obtained because even if the pulse frequency is increased, the laser gas deteriorated between the discharging electrodes by the electric discharge is completely removed before the next electric discharge is performed. At this time, the effect of increasing the pulse frequency is high, and the ability of the exposure device can be increased though the laser power decreases slightly.
And, the laser system need not be enlarged because the flow velocity between the discharging electrodes can be increased without enlarging neither the flowing fan nor the motor.
According to a second aspect of the invention, the ArF excimer laser described in the first aspect of the invention has Xe contained in the laser gas.
According to the second aspect of the invention, the ArF excimer laser having He as the buffer gas contains Xe in the laser gas.
There is an effect that the pulse output is increased by adding Xe to the laser gas. Besides, the addition of Xe provides an effect of easing deviations of the pulse output, and stabilized high laser power can be obtained.
And, the laser light with a high pulse frequency can be obtained without enlarging the device as described in the effect of the first aspect of the invention because He is used as the buffer gas.
Accordingly, the ability of the exposure device is enhanced, and exposure can be performed excellently.
A third aspect of the invention is a scanning type exposure device which performs exposure of an entire semiconductor chip on a wafer by moving the wafer while irradiating a pulsed laser light to each of a plurality of irradiation regions smaller than an area of the semiconductor chip, wherein a light source for oscillating the laser light is the ArF excimer laser described in the first or second aspect of the invention.
According to the third aspect of the invention, the ArF excimer laser having He as the buffer gas is used as the light source for the scanning type exposure device which performs the exposure of a single semiconductor chip by dividing it into a plurality of irradiation regions to expose the individual irradiation regions.
To perform the exposure by the scanning type exposure device, it is necessary to have the pulse output within a predetermined range to control the irregularity of the exposure due to deviations of the pulse output of the ArF excimer laser and to irradiate a predetermined number or more of laser pulses to each of the radiation regions. Therefore, this scanning type exposure device cannot increase the pulse output to decrease the number of irradiation pulses for each semiconductor chip as does the stepper and needs to increase the pulse frequency of the laser light in order to improve the exposure capacity per unit time.
The above scanning type exposure device can have high exposing capacity by using the ArF excimer laser of a high pulse frequency described in the first or second aspects of the invention as the light source for the laser light.
Besides, the pulse output of the laser light can have an improved stability to enable good exposure by using the ArF excimer laser having Xe added as the light source as described in the second aspect of the invention.
The scanning type exposure device irradiates the laser light to the irradiation region smaller than one semiconductor chip and therefore does not need a large projection lens as does the stepper. And it is desirable to produce a semiconductor chip having a large area.
To achieve the above second object, the ultraviolet laser device according to the fourth aspect of the invention is an ultraviolet laser device for oscillating pulsed laser by adding a trace quantity of xenon gas to gas for ultraviolet laser introduced into a chamber, and causing pulse oscillation in the chamber so as to excite the gas for ultraviolet laser to oscillate the pulsed laser, comprising: a xenon gas cylinder in which xenon gas is sealed; an ultraviolet laser gas cylinder in which the gas for ultraviolet laser is sealed; and ultraviolet laser gas supply piping for connecting the ultraviolet laser gas cylinder and the chamber, wherein a predetermined position of the ultraviolet laser gas supply piping and the xenon gas cylinder are connected by xenon gas piping.
The invention produces an effect that xenon gas can be fed into the chamber readily without modifying the existing laser gas supply equipment because it is configured to connect the xenon gas cylinder with the given point on the piping for supplying the gas for the ultraviolet laser, which connects the gas cylinder for the ultraviolet laser and the chamber, by the xenon gas piping. The added xenon gas is entered piping having a trace quantity, and a pressure gauge is used to measure the xenon gas, so that the xenon gas can be added to the laser gas easily and accurately. When the xenon gas cylinder is positioned in the neighborhood of the chamber, the chamber and the xenon gas cylinder can be changed simultaneously by closing a valve and removing a piping joint. Thus, a possibility that the impurity gas of the atmosphere etc. is introduced into the xenon measuring piping is decreased.
The ultraviolet laser device according to the fifth aspect of the invention is characterized in that a first valve and a second valve are disposed on the ultraviolet laser gas supply piping, a third valve is disposed on mixture piping formed by the first valve, the second valve and piping therebetween, and the third valve and the xenon gas cylinder are connected by the xenon gas piping.
According to the invention, the third valve is disposed on the mixture piping consisting of the first valve, the second valve and the piping between them, and the third valve and the xenon gas cylinder are connected by the xenon gas piping, so that the invention provides an effect that the xenon gas or the ultraviolet laser gas can be introduced easily into the chamber by merely opening and closing the first to third valves.
The ultraviolet laser device according to the sixth aspect of the invention is characterized in that the chamber is exhausted in a state that the second valve located on the mixture piping on the side of the ultraviolet laser gas cylinder and the third valve are closed, and the first valve located on the mixture piping on the side of the chamber is opened, thereafter the xenon gas is supplied into the mixture piping by closing the first valve and opening the third valve, and when it is measured that a gas pressure in the mixture piping has reached a predetermined gas pressure, the third valve is closed, and the first valve and the second valve are opened.
The invention closes the second valve and the third valve located on the mixture piping on the side of the ultraviolet laser gas cylinder, exhausts the chamber with the first valve located on the mixture piping on the side of the chamber opened, closes the first valve and opens the third valve to supply the xenon gas into the mixture piping, and when it is measured that the gas pressure in the mixture piping has become the predetermined gas pressure, closes the third valve and opens the first valve and the second valve. Thus, the invention produces an effect that the xenon gas or the ultraviolet laser gas can be introduced into the chamber with much ease by virtue of the vacuum of the chamber and the opening and closing of the first to third valves.